1. Field of the Invention
The present invention relates to a continuously variable power-branched transmission that can be operated at least partially, i.e., temporarily, in a power-branching operational mode. The present invention also relates to a method for operating such a transmission.
2. Description of the Related Art
Transmissions with continuously variable transmission ratio (CVT transmissions) are being used increasingly in vehicles, in particular in passenger cars, because of their convenience and the possible reduction of fuel consumption in comparison to stepped automatic transmissions that operate exclusively with planetary gear trains. Power is transmitted in such continuously variable transmissions by an endless torque-transmitting means that circulates between two pairs of conical disks, where an effective radius of each conical disk pair is variable by changing the spacing between the conical disks of a conical disk pair from each other. Therefore, in general a clamping pressure that is dependent on the torque being transmitted at the moment is produced by means of pressure pistons. Either a hydromechanical torque sensor is provided for that purpose or the clamping pressure is controlled freely by software. A combination of the two possibilities is also known from DE 103 02 992 A1. The clamping pressure increases principally with the detected torque, and depends exclusively on the transmission ratio. When the transmission ratio is to be changed, the torque-related pressure is superimposed with an adjusting pressure, by means of which the transmission ratio is changed.
For reasons of fuel consumption reduction, it is desirable to increase the spread of a transmission beyond the value that is possible with a single CVT transmission. That is achieved with so-called power-branched transmissions, in which the spread or transmission ratio range of a CVT transmission is utilized “twice,” in that the transmission ratio range of the CVT transmission is passed through twice over its entire spread range when changing the total transmission ratio of the power-branched transmission, for example in the opposite direction, by combining it with a system of gears and activating corresponding control links.
FIGS. 1 and 2 show an example of a power-branched transmission that is known per se and can be utilized in connection with the present invention.
An example of a drive engine of a vehicle is an internal combustion engine 2, which is connected through a start-up clutch 4 to an input shaft 6 of a power-branched transmission 8, whose output shaft is indicated by 10. The power-branched transmission 8 includes a variable speed drive unit 12 having a continuously variable transmission ratio, and at least one gear system 14, for example a planetary gear train, as well as at least two control clutches K1 and K2, whereby the variable speed drive unit 12 can be linked with the gear system 14 in various ways. Inputs of an electronic control and regulating device 16 are connected to an accelerator pedal sensor 18, a power control setting sensor 20 of the internal combustion engine 2, an engine speed sensor 22, a sensor 24 for an input shaft of variable speed drive unit 12, which can simultaneously be input shaft 6, a sensor 26 for detecting the speed of rotation of the output shaft of variable speed drive unit 12, a sensor 28 for detecting the speed of rotation of drive shaft 10, and possibly additional sensors. In the electronic control and regulating device 16, output signals are produced depending upon the signals from the various sensors and upon algorithms, performance data, etc., that are stored in the control and regulating device 16. A power setting element 30 of internal combustion engine 2, an actuator for start-up clutch 4, the torque-dependent pressure in pressure cylinders for the pairs of conical disks of variable speed drive unit 12, the pressures in adjusting cylinders for the conical disk pairs 12 for changing the transmission ratio of the latter, and control clutches K1 and K2 are controlled by the output signals. The figures do not show gear sets of the vehicle, on which rotational speed sensors can also be provided, as well as a clutch and/or a brake for reverse travel.
FIG. 2 shows an example of a power-branched transmission having a variable speed drive unit 12, one conical disk pair 30 of which one disk is connected to input shaft 6 with a rotationally fixed connection and can be coupled to a first gear 32 by means of a first control clutch K1. The other conical disk pair 34 of variable speed drive unit 12 is connected with a rotationally fixed connection to an output shaft 36, which in turn is connected with a rotationally fixed connection to the sun gear 37 of a gear system 14 in the form of a planetary gear train. The output shaft 36 can be coupled additionally through a second control clutch K2 to a second gear 38, which is rotationally engaged with first gear 32 through an intermediate gear 40. Second gear 38 is connected in a rotationally fixed connection to planet carrier 42 of the planetary gear train, whose planet gears 44 mesh with internal gear 46, which is connected to output shaft 10 in a rotationally fixed connection. When control clutch K2 is engaged and control clutch K1 is disengaged, sun gear 37 and planet carrier 42 rotate together, so that the planet gears 44 stand still and take internal gear 46 with them. The entire power-branched transmission then works like a simple CVT transmission, whose total transmission ratio is effectively doubled. At a shift point at which the transmission ratio of the variable speed drive unit has a small, predetermined value the control clutches K1 and K2 are reversed, so that the planet carrier 42 now turns with the transmission ratio corresponding to the transmission ratio that exists between the first gear 32, which is connected to input shaft 6 in a rotationally fixed connection, the intermediate gear 40, and the second gear 38, and the planetary gear system 14 becomes operative. The transmission ratios are chosen so that at a shift point U the total transmission ratio itotal of the power-branched transmission is independent of the shift state of control clutches K1 and K2. If the spread range of the variable speed drive unit 12 is passed through again with the control clutches K1 and K2 in that shift state, the transmission ratio itotal changes again.
Of course, it is also possible to design the transmission ranges so that the reversal is combined with a reduction of rotational speed caused by friction clutches. It is likewise possible to design the transmission ranges so that the reversal is combined with a change in the variable speed drive unit transmission ratio.
FIGS. 3, 4, 5 and 6 show examples of arrangements of power-branched CVT transmissions with various options for installing a torque detection means, for example a hydromechanical torque sensor or an electronic measuring device. If a hydromechanical torque sensor is used, it outputs a clamping pressure that corresponds to the conveyed torque. If an electronic measuring device is used instead, an electronic torque signal is delivered to the controller. Depending upon that torque, the movable conical disk of a disk pair is thus pressed toward the fixed disk to set the effective radius of the conical disk pair.
In FIGS. 3 through 6, Jengine, Jinput, JSS1, and JSS2, respectively, show schematically the moments of inertia corresponding to the engine shaft, the input shaft, and the variable speed drive unit shafts.
Reference numeral 50 designates the torque detection means, which either outputs clamping pressure directly, depending upon the torque, or it generates a corresponding signal as the input signal to the controller. FIGS. 3 through 6 show various non-limiting possibilities for power branching.
In such known power-branched continuously variable transmissions, the clamping force that is exerted on the conical disk pairs increases and decreases with the torque, and depends upon the transmission ratio.
Starting from that background, an object of the invention is to develop a further refinement of a continuously variable power-branched transmission, in particular for a motor vehicle, in such a way that wheel-side torques can be compensated for.